The present invention relates to an insulated-gate bipolar transistor (IGBT) as a power switching device applied to an inverter apparatus and so on.
An insulated-gate bipolar transistor (hereinafter referred to as IGBT) is a voltage-driven semiconductor switching device capable of high-speed on/off turning with a relatively low on-voltage, and is widely used in the field of power electronics including an inverter apparatus.
In the IGBT-output type inverter apparatus, an overcurrent flows through the IGBT at a starting-up of a motor or at an accident, such as load short-circuit or arm short-circuit. Therefore, strict requirements for withstanding high voltage and large current are imposed on the IGBT. Among the required items of the electrical characteristics, there is a breakdown withstanding capability called as a short-circuit withstanding capability.
On the other hand, the inverter apparatus includes a protection circuit that detects a short-circuit accident and shuts down the power. However, it takes about 10 to 20 micro seconds until the protection circuit starts to function from the overcurrent detection, so that the IGBT is required not to break during that period.
In view of the above, recent high-performance IGBT modules include an overcurrent protection system separately from the protection circuit of the above-described protection circuit. According to this overcurrent protection system, an overcurrent flowing through the IGBT at a short-circuit accident is quickly detected, and a signal based on the overcurrent detection is applied to the gate which controls the current flowing through the IGBT to be self-limited under or less than the short-circuit withstanding capability or capacity of the device before the power is shut down by the above protection circuit.
FIG. 7 shows an overcurrent protection circuit for an IGBT according to the above overcurrent protection system. In FIG. 7, reference numeral 1 denotes a main element (IGBT). A current detection auxiliary element 2, which is an IGBT different from the main element 1, is connected parallel to the main element 1. A current detection resistor 3 is connected in series to the auxiliary element 2. A switching element (MOSFET) 4, which turns on and off in accordance with a voltage appearing across the current detection resistor 3, is connected to gate drive circuits for both the main element 1 and the auxiliary element 2.
In the above structure, when an overcurrent flows through the main element 1 and the auxiliary element 2 due to an accident, such as a load short-circuit, and, as a result, a voltage appearing across the current detection resistor 3 exceeds a threshold voltage of the switching element 4, the switching element 4 is turned on to decrease the gate voltage of the main element 1 and the current detection auxiliary element 2, to thereby lower the main current flowing through the main element 1 that is the subject to be protected. In this case, the main current flowing through the IGBT as the subject to be protected can be made smaller than the short-circuit withstanding capability or capacity of the IGBT by properly setting the resistance of the current detection resistor 3 and the threshold voltage of the switching element 4.
Short-circuit of an inverter includes accidents, such as arm short-circuit, series short-circuit, output short-circuit, grounding short-circuit and so on. In order to perform a reliable protection by quickly detecting an overcurrent flowing though the main IGBT element 1 with high accuracy, it is very important that the ratio of the main current in the main element 1 to the detection current in the auxiliary element 2 is always kept constant irrespective of the above various kinds of short-circuit modes. Further, the collector-emitter voltage V.sub.CE applied to the IGBT to be protected is expected to vary with the difference in the short-circuit mode in the load short-circuit accidents of an inverter. Then, the above-described overcurrent protection circuit must limit the overcurrent under the short-circuit withstanding capacity of the IGBT so that the limited current has little dependency on the collector-emitter voltage V.sub.CE with little variation.
In the conventional protection circuit in which the current detection auxiliary element 2 is provided separately from the main element 1, it is technically difficult to make operation characteristics of the main element 1 and the auxiliary element 2 proportional to each other. In particular, when the collector-emitter voltage V.sub.CE of the IGBT varies as described above, the current ratio between the main element 1 and the auxiliary element 2 varies and, accordingly, the limited current value also varies greatly. Thus, it is difficult to establish a reliable overcurrent protecting operation.
The present invention has been made in view of the above circumstances, and has an object of providing an insulated-gate bipolar transistor with an overcurrent limiting function, which provides reliable overcurrent protection by keeping the ratio between the main current and the detection current constant under various operation environments, and suppressing a collector-emitter voltage dependency and a variation of the limited current.